1. Field of the Invention
The present invention relates to a method and apparatus for receiving orthogonal frequency division multiplexed (OFDM) signals, and more particularly, to a method and apparatus for timing and frequency synchronization of an OFDM signal receiver to an OFDM signal.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) is a robust technique for efficiently transmitting data using a plurality of sub-carriers within a channel bandwidth. These sub-carriers are arranged for optimal bandwidth efficiency compared to more conventional transmission approaches, such as frequency division multiplexing (FDM). FDM separates and isolates the sub-carrier frequency spectra, and requires a frequency guard band to avoid inter-sub-carrier interference, thereby increasing overhead and degrading bandwidth efficiency.
By contrast, although optimal bandwidth efficiency is obtained by overlapping the frequency spectra of OFDM sub-carriers, the OFDM sub-carriers must remain orthogonal to one another to prevent interference between sub-carriers. Additionally, an OFDM symbol is resistant to multipath fading because it is significantly long compared to the length of the channel impulse response and inter-symbol interference can be completely prevented.
FIG. 1 is a block diagram of a typical OFDM signal transmitter. An encoder 110 encodes a stream of input data bits bn and outputs a stream of sub-symbols Xn. An inverse fast Fourier transformer (IFFT) 115 performs an N-point inverse discrete Fourier transformation (IDFT) or inverse fast Fourier transformation (IFFT) on the stream of sub-symbols Xn. Here, n denotes a frequency-domain index, and also can denote a sub-carrier index. N sub-symbols Xn are equivalent to one frequency-domain OFDM symbol, and they are typically phase shift keyed (PSK) signals or quadrature amplitude modulated (QAM) signals.
A frequency-domain OFDM symbol is usually designated as zero at a zero-frequency DC and around the edges of a passband, as shown in FIG. 2. Accordingly, a transmitter/receiver can easily perform analog filtering, and the influence of noise on a received signal is reduced. IFFT 115 transforms the frequency-domain OFDM symbol into a time-domain symbol according to the following Equation 1:                                           x            K                    =                                    1              /              N                        ⁢                                          ∑                                  n                  =                  0                                                  N                  -                  1                                            ⁢                                                X                  n                                ⁢                                  ⅇ                                      j                    ⁢                                          xe2x80x83                                        ⁢                    2                    ⁢                                          nkn                      /                      N                                                                                                          ,                  xe2x80x83                ⁢                  k          =          0                ,                              …            ⁢                          xe2x80x83                        ⁢            N                    -          1                                    (        1        )            
wherein xk denotes samples of a time-domain OFDM symbol, and k is a time-domain index.
A digital signal processor (DSP) 120 adds a cyclic prefix or guard interval of G samples before N samples, i.e., a sequence of the output of IFFT 115. Thus, one time-domain OFDM symbol is comprised of (G+N) samples, as shown in FIG. 3. The cyclic prefix is comprised of the last G samples among the output of IFFT 115. This cyclic prefix is typically longer than the channel impulse response and, therefore, acts to prevent inter-symbol interference between consecutive OFDM symbols.
The output of DSP 120 is divided into real and imaginary-valued digital components. The real and imaginary-valued digital components are then passed to digital-to-analog converters (DACs) 130 and 135, respectively. DACs 130 and 135 convert the real and imaginary-valued digital components into analog signals at a sampling frequency of fs=1/Ts Hz as determined by a clock circuit 125. The analog signals pass through low pass filters (LPFs) 140 and 145 and become in-phase and quadrature OFDM analog signals, respectively. The in-phase and quadrature OFDM analog signals are then passed to mixers 160 and 165.
As a result of the above IFFT, D/A conversion, and low pass filtering, N sub-symbols in the OFDM symbol are transmitted by being carried on N sub-carriers. As shown in FIG. 4, the sub-carriers each display a sinc(x)=sin(x)/x spectrum in the frequency domain, and the peak frequencies of the sub-carriers are spaced fs/N=1/NTs Hz apart from each other. Here, when the time for N samples in one OFDM symbol is T. T is equal to NTs. Also, although the spectra of the sub-carriers overlap, a given sub-carrier remains orthogonal to neighboring sub-carriers because neighboring sub-carriers become null at the peak of the given sub-carrier.
In mixers 160 and 165, the in-phase and quadrature OFDM analog signals from LPF 140 and 145 are mixed with an in-phase intermediate frequency (IF) signal and a 90xc2x0 phase-shifted IF signal, respectively, in order to produce an in-phase IF OFDM signal and a 90xc2x0 phase-shifted (quadrature) IF OFDM signal, respectively. The in-phase IF signal fed to the mixer 160 is produced directly by an IF local oscillator (Lo) 150, while the 90xc2x0 phase-shifted IF signal fed to the mixer 165 is produced by passing the in-phase IF signal produced by Lo 150 through a 90xc2x0 phase-shifter 155 before feeding it to mixer 165. These two in-phase and quadrature IF OFDM signals are then combined in a combiner 167, and the combined IF OFDM signal is transmitted via a radio frequency (RF) signal transmitter 170.
The RF signal transmitter 170 includes a bandpass filter (BPF) 175, an RF mixer 183, an RF carrier frequency local oscillator (Lo) 180, another BPF 185, an RF power amplifier 190, and an antenna 195. The combined IF OFDM signal from combiner 167 is filtered by the BPF 175, and shifted by the frequency of the Lo 180 by the mixer 183. The frequency-shifted signal is again filtered by the BPF 185, amplified by the RF power amplifier 190, and finally transmitted via the antenna 195. When the sum of the frequencies of the Lo 150 and the Lo 180 is fc for conveniencexe2x80x2 sake, fc becomes the central frequency of a passband signal, i.e., a carrier frequency. The frequency fs of the clock circuit 125 determines the bandwidth of a transmitted signal and the sub-carrier frequency interval.
A receiver for receiving signals transmitted through the above-described process and restoring original data bits is essentially configured such that its component units are arranged opposite to those of the transmitter. FIG. 5 is a block diagram of the configuration of a typical OFDM signal receiver. An RF receiver 210 usually includes an antenna 212, a low noise amplifier 215, a bandpass filter BPF 217, an automatic gain controller (AGC) 220, an RF mixer 222, an RF carrier frequency local oscillator (Lo) 225, and an IF BPF 227. The low noise amplifier 215 amplifies an RF signal received from the antenna 212. BPF 217 bandpass-filters the amplified RF signal. AGC 220 automatically keeps the magnitude of the filtered signal at a predetermined magnitude. The mixer 222 converts the RF signal into an IF signal, and BPF 227 bandpass-filters the output of the mixer 222 and passes only a desired IF signal. Lo 225 determines the degree of frequency shifting when the RF signal is converted into the IF signal by mixer 222.
The IF signal output from the BPF 227 is converted into an analog baseband in-phase signal and an analog baseband quadrature signal while passing through mixers 230 and 235 and LPFs 250 and 255. An Lo 240 determines the degree of frequency shifting when the IF signal is converted into baseband signals. Analog-to-digital converters (ADCs) 260 and 265 convert the output signals of LPFs 250 and 255 into digital signals, respectively. The operational frequencies of the ADCs 260 and 265 are determined by the frequency of a clock circuit 270.
A DSP 275 removes a cyclic prefix added to each OFDM symbol from a complex sample signal rk of the output signals of ADCs 260 and 265, finds the FFT start position, and outputs N samples to an FFT 280. FFT 280 performs a fast-Fourier-transformation on the cyclic prefix-removed signal, and outputs a frequency domain signal Rn. Rn is expressed by the following Equation 2:                                           R            n                    =                                    ∑                              k                =                0                                            N                -                1                                      ⁢                                          r                k                            ⁢                              ⅇ                                                      -                    j                                    ⁢                                      xe2x80x83                                    ⁢                  2                  ⁢                                      xe2x80x83                                    ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                                      kn                    /                    N                                                                                      ,                  xe2x80x83                ⁢                  n          =          0                ,        …        ⁢                  xe2x80x83                ,                  N          -          1                                    (        2        )            
A detector/decoder 285 detects an originally-transmitted sub-symbol from Rn, decodes it, and outputs a binary data sequence.
In the OFDM signal transmission and reception as described above, the receiver must be exactly synchronized with the transmitter. The synchronization will now be described.
First, the receiver finds the exact FFT starting point of each OFDM symbol, removes a cyclic prefix from each OFDM symbol, and performs an FFT. If the receiver does not detect the correct FFT starting position of each OFDM symbol from a received signal, data detection error is increased by the interference between adjacent OFDM symbols during output of a fast-Fourier-transformed signal.
Second, the sum fcxe2x80x2 of local oscillator frequencies in the receiver must be the same as the sum fc of the local oscillator frequencies in the transmitter. Here, fc is the same as the carrier frequencies of a transmitted signal. If fcxe2x80x2 is not exactly consistent with fc, a frequency offset xcex94fc=fxe2x80x2xe2x88x92fc exists in the received complex signal rk. Since an OFDM signal is very sensitive to this frequency offset, interference between sub-carriers is generated in a received signal. Thus, data detection failure suddenly increases.
Third, the clock frequency fsxe2x80x2 supplied to the ADC in the receiver must be the same as the clock frequency fs supplied to the DAC in the transmitter. When the sampling clock frequencies are not the same, the frequency-domain signal is proportional to the frequency index. Thus, the interference between sub-carriers increases, and the phases of sub-symbols vary, thereby increasing data errors.
In the U.S. Pat. No. 5,732,113, two reference symbols are used to accomplish the symbol timing synchronization, the carrier frequency synchronization, and the sampling clock synchronization.
To solve the above problem, it is an objective of the present invention to provide an OFDM receiver synchronizing method and apparatus for accomplishing symbol/frame timing synchronization, carrier frequency synchronization, and sampling clock frequency synchronization with respect to an OFDM signal using one reference symbol.
Accordingly, to achieve the above objective, there is provided a method of synchronizing an OFDM receiver to an OFDM signal comprising the steps of: (a) receiving an OFDM reference symbol before user data OFDM symbols, the OFDM reference symbol having sub-symbol signals at only random, even-numbered sub-carriers and having no signals at any other even-numbered sub-carriers and all odd-numbered sub-carriers in the frequency domain, and the first half of the symbol having the same characteristics as the remaining half in the time domain; (b) obtaining OFDM digital signal samples rk by sampling the received OFDM reference symbol and converting the OFDM reference symbol samples into digital signals; (c) obtaining a predetermined timing metric for each of the OFDM signal samples rk according to the time-domain characteristics of the reference symbol, and detecting the point in time at which the power of the timing metric is maximum as the starting point of a symbol/frame of the OFDM signal; and (d) correcting for the offsets of sub-carrier frequencies that are less than the frequency spacing between two adjacent sub-carriers by obtaining a frequency offset from the phase of the timing metric at the detected symbol/frame starting point, and correcting for the offsets of sub-carrier frequencies that are greater than the frequency spacing between two adjacent sub-carriers by obtaining the value which maximizes the power metrics according to the frequency-domain characteristics of the reference symbol.
To achieve the above objective, there is provided an apparatus for synchronizing an OFDM receiver, comprising: an OFDM signal receiving portion for receiving an OFDM reference symbol before user data OFDM symbols, the OFDM reference symbol having sub-symbol signals carried by random, even-numbered sub-carriers and having no signals carried by the other even-numbered sub-carriers and all odd-numbered sub-carriers in the frequency domain, and the first half of the symbol has the same characteristics as the second half in the time domain; an analog-to-digital converter (ADC) for sampling the received OFDM reference symbol, converting the OFDM reference symbol samples into digital signals, and outputting OFDM digital signal samples rk; and an operation processor for storing the OFDM digital signal samples rk in an internal buffer, detecting the starting point of a symbol/frame which satisfies a predetermined timing metric, from the time-domain characteristics of the stored samples, obtaining a carrier frequency offset from the phase of the metric at the detected starting point and correcting for the offset, and obtaining the sampling period error of the ADC from the phase difference generated when the stored N samples are transformed into two N/2 frequency-domain symbols and correcting for the error.